The present invention relates to the process of imaging the anatomy and physiological properties of a tissue, and in particular, it concerns a method for imaging the histological layers of a blood vessel wall and mapping the tissue strain in those layers.
It is known that cardiovascular disease is the single most important cause of mortality in first world populations today, primarily due to disease processes causing narrowing and/or disruption of the vessel wall of the coronary and other major arteries. Investigation of the mechanical properties and structure of human blood vessel walls is thus an important field of laboratory and clinical research.
Blood vessel walls consist of three histological tissue layers, any of which may be involved in a pathological process: the outer layer, the tunica adventitia, the intermediate layer, the tunica media and the inner layer--the tunica intima. Each of these layers posses a unique biochemical and mechanical structure, alterations in which may be associated not only with pathological processes, but with normal physiological variants as well. In addition, it is known that at normal blood pressure the length of a vessel is as much as 40% longer, and its circumference up to 30% greater, than that in the unstressed condition. The layers of a blood vessel wall are thus exposed to significant stress and strain in normal physiological conditions, and alterations in vessel wall strain may conceivably be an important cause of, or consequence, of, vessel wall pathology. The standard imaging modalities for investigating blood vessels (contrast angiography and magnetic resonance angiography), however, do not image the blood vessel wall itself, but rather the contents of the vessel lumen: the blood. Other cardiovascular imaging techniques such as conventional ultrasound and standard T1 or T2 weighted magnetic resonance imaging (MRI) do not have sufficient resolution to adequately image the tissues constituting the vessel wall, as human blood vessel walls are very thin (approximately 2 millimeters thick). Most importantly, all the above mentioned cardiovascular imaging techniques suffer from two major deficiencies: firstly, the vessel wall can only be imaged (if at all) in its entirety, as a single unit, making accurate differentiation between the three tissue layers difficult; and secondly, no information describing the biochemical and mechanical properties of the vessel wall (such as the tissue strain in each histological layer) is provided.
There is thus a need for an imaging method capable of accurately depicting the tissue layers constituting a blood vessel wall, and quantifying their biochemical and mechanical characteristics. The present invention is a method for displaying the distinct tissue layers of a blood vessel wall and quantifying their mechanical strain, based on a new technique of magnetic resonance imaging: .sup.1 H or .sup.2 H double quantum filtered MRI.
The basic principles underlying the process of magnetic resonance spectroscopy and imaging will be well known to one familiar with the art. In brief, nuclei possessing an odd number of protons (such as hydrogen .sup.1 H) possess a magnetic moment, and behave in a predictable manner when exposed to a strong external magnetic field (Bn). Such nuclei will align themselves relative to the external magnetic field and rotate on their axes, or precess, in a manner which is dependent on the individual nuclei chemical and physical surrounding and the orientation of the applied external and magnetic field. Then, if exposed to a pulse of radio frequency (RF) energy of appropriate frequency, timing and duration, the nuclei will absorb such energy by changing he orientation of their magnetic moments relative to each other and the external magnetic field, leading to a change in the nature of their precession. The energy absorption (or emission) is the well known phenomenon of magnetic resonance. When the RF pulse is discontinued, the nuclei emit a detectable radio frequency signal (as the nuclei "relax" back to their prior energy state by non-radiative processes) typical of the specific nuclei and its specific chemical and physical surrounding. The emitted signal, known as the "free induction decay" (FID), depicts a change of magnetization over time, and can also be described in terms of a superposition of many RF frequency. These time characteristics and RF characteristics of the FID can be analyzed so as to obtain information about the properties of the nuclei being studied. The transition from the time domain (the FID) to the frequency domain (the RF spectrum), and vice-versa, is achieved by performing a fast Fourier transform and its inverse. Thus, any calculation performed in the frequency domain will have an analogous calculation in the time domain, and vice-versa.
The technique of magnetic resonance spectroscopy described above furnishes magnetic resonance data about the microscopic environment Spectroscopy in most of its application does not involve spatial localization. Thus it provides information about the physics and chemistry of the constituents of a tissue, but does not present an image. However, by exposing the tissue being studied to appropriately timed magnetic gradients of varying intensity along the spatial axes, known as "phase encoding gradients", the spatial location of the nuclei giving rise to the FID can be localized in three dimensions, thus allowing for two or three dimensional mapping, or imaging, of the tissue based on the MR signal, i.e. a process of magnetic resonance imaging (MRI).
Standard MRI techniques (such as gradient echo imaging and spin echo imaging) are based on measuring the FID emitted by hydrogen nuclei .sup.1), which are simple magnet dipoles possessing a spin of 1/2. In the presence of an external magnetic field (B.sub.0) such dipoles can be described as being in one of two energy states (described by the magnetic quantum numbers -1/2, or +1/2), and when excited by an appropriate RF pulse, the nuclei undergo "single quantum" transitions between these two states. These single quantum transition are responsible for the creation of a detectable FID.
More complex nuclei, such as deuterium (.sup.2 H), are quadrupolar. Such nuclei have a spin &gt;1/2, may be described as being in one of multiple possible energy states. In the case of deuterium, the possible energy states are described by the magnetic quantum numbers -1,0 and +1.If transitions occur between energy states -1 and +1, as facilitated by an appropriate RF pulse, the nuclei are said to have undergone "double quantum" transitions.
When quadrupolar nuclei are located in a disordered environment, such as a liquid, the rapid molecular reorientations of the nuclei are "isotropic", meaning that the reorientations of each molecule are such that overall directional information averages to zero. As such, the occurrence of double quantum transitions is not detectable. However, when quadrupolar nuclei are bound to an ordered protein structure, such as the collagen fibres of a blood vessel wall, the rapid movements of the quadrupolar nuclei demonstrate a directional tendency. Such movement is thus "anisotropic", and in terms of the electrical interaction between a quadrupolar nucleus and its electrical environment, results in the formation of a "non-vanishing residual quadrupolar interaction", which facilitates the detection of a double quantum transitions.
The occurrence of a double quantum energy transition is associated with the formation of a "double quantum coherence" state. Although such double quantum coherences cannot be directly observed by nuclear magnetic resonance techniques, the application of additional appropriate RF pulses to the tissue can transform the double quantum coherences into a detectable signal quantum coherence. The prior existence of a double quantum coherence in the tissue being studied can be revealed by specific characteristics of the resultant FID: a first order effect of residual quadrupolar splitting (i.e. the presence of peaks of two frequencies), and, in a second order approximation, an increased rate of relaxation due to rapidly fluctuating electric field gradients, caused by the molecular motion of the quadrupolar nuclei. The detection of residual quadrupolar splitting in an FID is thus indicative of the presence of anisotropy, and thus order, within the tissue being studied. Furthermore the degree of residual quadrupolar splitting is proportional to the degree of order within the tissue. Similarly, measurement of the second moment of the distribution of quadrupolar splitting of the FID may allow for an evaluation of the degree of order in the tissue.
When exposed to deuterated water (D.sub.2 O), the hydrogen (.sup.1 H) nuclei in the water of a biological tissue are replaced by deuterium (.sup.2 H) resulting in deuterated water becoming bound with the protein skeleton (mainly the collagen fibres) of the tissue. Due to the order in the protein skeleton, the bound deuterated water exhibits the properties of anisotropy described above. As such, deuterium can serve as a "spy" molecule allowing for the invention of the degree of anisotropy of a tissue.
When deuterated water is abundantly present in close proximity to an ordered tissue being imaged, as it usually the case with biological tissues, the magnetic resonance signal originating from the liquid masks the signal indicative of a non-vanishing residual quadrupolar interaction. In such circumstances filtering of the magnetic resonance signal is necessary for selective interrogation of the anisotropic tissue, such that only those FID signals which initially originated from double quantum coherences are preserved. Thus, by filtering out FID signals not originating from double quantum coherences, the renaming FID frequency and time domain data provide information regarding the degree of anisotropy, i.e. order, in the tissue to which the quadrupolar nuclei are bound. This is the basic principle underlying double quantum filtered (DQF) magnetic resonance spectroscopy.
It is known that as a tissue is stressed (for example, by being stretched) the strain within the tissue increases. It has also been shown that with increasing strain, the degree of order within the protein skeleton of the tissue increases. As shown above, the degree of order (anisotropy) can be quantified from the degree of quadrupolar splitting in a DQF MR signal. If the correlation between the degree of quadrupolar splitting and the degree of tissue strain for a given tissue is known, therefore, DQF MR spectral data can be used to quantiate the degree of tissue strain (Y. Sharf, S. Akselrod, G. Navon, "Measurement of strain exerted on blood vessel walls by double-quantum-filtered .sup.2 H NMR, " Magnetic Resonance in Medicine. 1997:37:69-75). .sup.2 H DQF MR spectroscopy of this nature has been performed on blood vessels and other biological tissues (Y. Sharf, U. Eliav, H. Shinar, G. Navon, "Detection of anisotropy in cartilage using .sup.2 H double-quantum-filtered NMR spectroscopy," Journal of Magnetic Resonance. 1994;B107:60-67). However, as spectroscopy alone does not furnish spatial information, this technique does not allow for two dimensional mapping of vessel wall anatomy or vessel wall strain.
A method for two dimensional mapping of strain in elastomer bands made of rubber, utilizing an .sup.2 H DQF MRI technique, has been previously reported (M. Klinkenberg, P. Blumler, B. Blumich, ".sup.3 H NMR imaging of stress in strained elastomers," Macromolecules 1997;30:1038-1043). Several factors, however, make this technique for imaging non-biologic materials unsuitable for use in biological tissues:
1. The technique used single quantum MR sequences (spin echo sequences) for calibration of the .sup.2 H DQF MRI data. This is ineffective as a means of calibrating .sup.2 H DQF MRI quadrupolar splitting data to tissue strain data, for the purpose of generating a tissue strain map, as single quantum techniques do not suppress the abundant MR signal generated by isotropic free water (or deuterated water) in biologic tissues. Investigation of anisotropic biologic tissues is thus not possible using this single quantum based technique.
2. The technique generated a single frequency value for each voxel, not a full FID spectral signal as is necessary for a quantitative evaluation of anisotropy. As such, the .sup.2 DQF MRI strain maps generated by this technique were qualitative, and not quantitative, in nature. In addition, the residual quadrupolar splitting cannot be directly measured in the absence of a full FID spectrum. The technique therefore required acquiring multiple sets of images using the same imaging sequence with a different creation time (.tau.) value, and then fitting the resultant data to a theoretical model so as to obtain a value for quadrupolar splitting. This process of fitting the data to a theoretical model introduces a degree of inaccuracy not present when the quadrupolar splitting is acquired directly from the .sup.2 H DQF FID signal.
3. In order to couple deuterium to the network chains of interest in the rubber, deuterated poly(butadiene) oligomers were used. As this compound is hydrophobic, it is not suitable for use in biological tissues. Furthermore, as the interaction of oligomers with rubber is not analogous to that of deuterated water with biologic tissues, performance of .sup.2 H DQF MRI strain mapping on rubber does not imply that the same technique of .sup.2 H DQF MRI strain mapping is possible on biological tissues.